Heretofore, there has been a shank attaching structure of the so-called “screw-locking type” adapted to secure the shank of a cutter to an arbor attached at its base end to a rotating shaft of a drilling apparatus (an electric hand drill or a drilling machine) by side locking with a screw. At present, this screw-locking type shank attaching structure is most prevalent on a worldwide scale. Such a screw-locking type shank attaching structure, however, offers a low operability because it requires that the screw be loosened and fastened for detachment and attachment of cutters with use of a tool such as a hexagonal wrench at every change of the diameter of a hole to be drilled. In an attempt to solve such a problem, a shank attaching structure has been provided of a construction capable of detachably attaching a shank to an arbor wherein an attachment/detachment ring for attachment and detachment of the shank is provided which is operable (rotatable or slidable) to cause a lock member protruding into the shank attaching bore of the arbor to be retracted from the shank attaching bore thereby to release the locking between the lock member and an engaging portion provided at the outer periphery of the shank of a cutter (Japanese Patent Laid-Open Gazette No. HEI 10-537512).
Such shank attaching structures adapted to attach a shank to an arbor by operating this attachment/detachment ring include: those of the so-called “one-touch type” capable of attaching the shank to the arbor without requiring any operation of an worker on the attachment/detachment ring; and those of the so-called “two-touch type” adapted to attach the shank to the arbor by operating the attachment/detachment ring. Either type involves technical problems to be described below. First, in either case of the one-touch type shank attaching structure and the two-touch type shank attaching structure, the shank of a cutter axially freely moves within a very small dimension relative to the arbor due to dimensional tolerances (including machining tolerances, assembling tolerances and the like). Specifically, though the shank is positioned by causing the lock member on the arbor side to engage the engaging portion on the shank side at the time of attachment as described above, the shank of the cutter axially freely moves within a very small dimension relative to the arbor due to so-called “play” between the lock member and another portion of the arbor holding the lock member and “play” between the lock member and the engaging portion on the shank side. Accordingly, in the case where a drilling operation is performed using a drilling apparatus provided with a chip breaking mechanism which provides fluctuation in the axial direction to break chips into fragments, the aforementioned free movement of the shank offsets or considerably attenuate the chip breaking operation. For this reason, the chip breaking mechanism does not function effectively and hence cannot break strip-shaped chips produced during a cutting operation. It should be noted that the aforementioned chip breaking mechanism refers to a mechanism such as to vary or stop the feed of a cutter in the axial direction or reverse the feed direction at appropriate intervals thereby breaking strip-shaped chips produced by the cutting edge into fragments.
Secondly, the prior art one-touch type shank attaching structure is constructed so that a supporting member restraining the lock member from protruding into the shank attaching bore of the arbor is pressed from the base end of the arbor with a spring to position laterally of the lock member. The construction of this portion is intricate, which increases a parts count. Further, since the shank attaching structure needs to be assembled while positioning the supporting member urged to spring out of the shank attaching bore toward the opening thereof and the spring pressing against the supporting member, which are kept in their restrained states, such assemblage is complex and hence requires a skill.
Thirdly, in drilling a hole with a cutter attached to a one-touch or two-touch type shank attaching structure of this type, when a rotating torque, particularly an impactive rotating torque, is exerted during the drilling operation, the edge of the engaging portion of the shank is deformed to project radially outwardly by the lock member, thus resulting in such an inconvenience that the shank cannot be drawn out of the arbor due to lodging of this projecting portion. In such a case the drilling operation is interrupted for a long time, which incurs a great economic loss. Free movement of the shank relative to the arbor due to the aforementioned dimensional tolerances takes place also in the rotational direction (circumferential direction) of the shank. In such a case, particularly, the dimension of such a deformation is enlarged.